Chill-casting of metal foils is a process whereby a stream of molten metal is impinged upon a rapidly moving cooled surface which not only chill quenches a layer of molten metal but translates this layer away from the impingement region to produce continuous foil.
These foils are generated by the flow of molten metal through a very fine orifice(s) or slot(s) which are easily plugged by particles in the molten metal such as an oxide particulate. A variety of metals and metal alloys form an oxide when exposed to air and this formation is accelerated when the metal is heated to a molten state. The oxide formation can inhibit the flow of the molten metal during the melt spinning process by partially or completely blocking the reservoir orifice. Therefore, it is important to prevent air from coming in contact with the molten metal to prevent an oxide formation.
One of the conventional methods for preventing oxidation uses a purging gas stream that flows around the metal charge inside the reservoir (e.g. crucible) prior to the melting step as well as flushing a purge gas around the reservoir orifice externally. This approach suffers severe limitations.
In order to prevent the molten metal from prematurely flowing through the orifice of the reservoir, the purging gas stream must be shut off prior to melting the metal charge, and depending upon the diameter of the orifice a slight negative pressure may have to be applied to the reservoir to prevent the metal once melted from flowing through the orifice. An external application of purge gas is not very effective to prevent oxidation of the molten metal since during the melt spinning process the chill wheel rotates at very high speeds, typically 3000 ft/min, which generates a very strong air boundary layer on the surface of the wheel and forces air into the external purging gas shroud and into the orifice thereby exposing the molten metal to air prior to the initiation of the molten metal flow. This exposure can cause oxidation of the molten metal and a formation of particulates which are detrimental to high quality foil processing. Because of these problems, many oxidation prone materials are processed inside isolation chambers where air could be completely eliminated. However, this approach does not lend itself to larger scale production operations.
A new and improved apparatus and method were developed to minimize these problems.